class CSPDarknet53_P2(nn.Module):
    def __init__(self):
        super().__init__()
        self.stem = Stem()  # 3->64, stride=2
        self.stage1 = Bottleneck(64, 128, n=3)  # /2
        self.stage2 = Bottleneck(128, 256, n=6) # /4 → P2输出
        self.stage3 = Bottleneck(256, 512, n=9) # /8
        self.stage4 = Bottleneck(512, 1024, n=3) # /16